1. Field of the Invention
The present invention relates to hydride compositions. In particular, the present invention relates to a metallic hydride composition that can undergo repeated hydrogen absorption/desorption cycles without disintegrating, and a process for making such a composition.
2. Discussion of Background
Metal hydrides are capable of absorbing large amounts of hydrogen which can then be desorbed under the appropriate temperature and pressure conditions. These materials have many applications, particularly in the hydrogen processing and energy conversion fields. They are used for hydrogen storage, hydrogen pumping and compression, heat pumps, batteries, and fuel cells. Hydrides are selective in that they only absorb hydrogen, and also differentially absorb the three isotopes of hydrogen (protium, deuterium, and tritium). Thus, hydrides are useful in hydrogen isotopes purification and separation.
Known hydride formers (loosely called hydrides) include pure metals (Mg, Ti, V, Nb, Pt, Pd, and so forth) and alloys (the La-, Ti-, and Co- alloys, and rare earth-Ni alloys). The capacity of a particular material to absorb or release hydrogen depends on the temperature, the external hydrogen gas pressure, and the surface area of the material. To maximize the surface area and the absorption/desorption efficiency, the hydride is often supplied in the form of small-grained particles or pellets.
In typical applications, the hydride particles are subjected to repeated absorption/desorption cycles. With each cycle, the particle size is reduced until, eventually, the particles disintegrate into a fine-grained powder. The powder forms a dense compact that is not readily permeable to hydrogen, so the efficiency of the process is reduced. It is a poor heat conductor, further reducing the efficiency of both the absorption and desorption phases of the cycle. The compacted powder can expand during the absorption phase, potentially damaging the container. Furthermore, the powder can readily become entrained in the gas stream, migrating and causing contamination of downstream piping and equipment. Even if filters are used, the fine mesh required for the small particles is easily clogged.
Various hydride compositions have been developed in attempts to alleviate the problems associated with pure hydrides. A metal hydride may be mixed with a non-hydridable ballast or matrix metal, as disclosed by Goodell, et al. (U.S. Pat. No. 4,589,919) and Ron et al. (U.S. Pat. No. 4,292,265). Hydride particles may be contained in a polymeric composition, such as the compositions described by Bernstein, et al. (U.S. Pat. No. 4,433,063), Buhl, et al. (U.S. Pat. No. 4,110,425), and Blytas (U.S. Pat. No. 4,036,944).
Numerous methods and processes are known for making hydride compositions. Leppard (U.S. Pat. No. 4,459,270) discloses a process for removing hydrogen from an oxygen-containing wet air stream by contact with a hydride composition. The composition is prepared by dry blending tin oxide with 10-50% alumina trihydrate. The mixture is pressed into pellets that are dried, calcined, and impregnated with 0.25-2.5 wt.% each of Pt and Pd. The pellets are dried and calcined in air, reduced in a gas stream of 5% hydrogen and 95% nitrogen, and cooled in nitrogen. When cool, a gas mix containing 1-10% hydrogen in nitrogen is flowed over the pellets.
Helversen (U.S. Pat. No. 4,249,654) discloses a hydrogen storage container having a hydrogen-storing material therein. The material comprises particles of a hydride-forming metal coated on the surface of a diatomaceous earth or other rare earth, porous ceramic, or glass fiber.
A stable hydrogen-absorbing composition and a method for making such a composition are described in commonly assigned and recently filed patent application Ser. No. 07/933,152, filed Aug. 21, 1992, titled Palladium/Kieselguhr Composition and Method. The composition is made by immersing a porous substrate such as kieselguhr in a concentrated solution of tetra-amine palladium (II) nitrate. Palladium from the solution is deposited onto the substrate, which is then removed from the solution, dried, and calcined. This process is repeated until the desired amount of palladium has been deposited onto the substrate. Finally, the material is heat treated to ensure reduction of the palladium to metal.
Hydride compositions such as these, while withstanding repeated cycling better than pure hydrides, eventually disintegrate into a fine powder after a large number of absorption/desorption cycles. Many of these more stable compositions have relatively low hydride content and lower hydrogen-absorbing capability than a pure hydride.
There exists a need for a dimensionally stable hydride composition with a high hydride content that can reversibly absorb large amounts of hydrogen. The composition should have sufficient porosity to allow permeation of hydrogen gas, and maintain its hydrogen-absorbing capacity over a large number of absorption/desorption cycles.